The present invention relates generally to downhole sealing systems for use in subterranean wells.
In the drilling and completion of oil and gas wells, a great variety of downhole tools are used. For example, but not by way of limitation, it is often desirable to seal tubing or other pipe in the casing of the well. Downhole tools referred to as packers and bridge plugs are designed for these general purposes and are well known in the art of producing oil and gas.
When it is desired to remove many of these downhole tools from a wellbore, it is frequently simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the packer or plug, for example, or at least the outer components thereof, out of the wellbore. Milling is a relatively slow process, but milling with conventional tubular strings can be used to remove packers or bridge plugs having relative hard components such as erosion-resistant hard steel.
In drilling, a drill bit is used to cut and grind up the components of the downhole tool to remove it from the wellbore. This is a much faster operation than milling, but requires the tool to be made out of materials which can be accommodated by the drill bit.
Such drillable devices have worked well and provide improved operating performances at relatively high temperatures and pressures. A number of U.S. patents in this area have been issued to the assignee of the present invention, including U.S. Pat. Nos. 5,224,540; 5,271,468; 5,390,737; 5,540,279; 5,701,959; 5,839,515; and 6,220,349, which are hereby incorporated by reference herein in their entirety. However, drilling out hardened iron components may require certain techniques to overcome known problems and difficulties. The implementation of such techniques often results in increased time and costs.
Improvements in the area of drillable downhole tools are still needed and the present invention is directed to that need.